The technical field of this invention is pyroelectric imaging devices and, in particular, image modulators for such devices.
It is well known that pyroelectric type detectors exhibit an output signal proportional to the rate of change of the input radiation rather than to the value of such radiation. As a result, in order to obtain detection of time-invariant radiation, it is necessary to modulate the latter. This is done by panning the detector with a cyclic vibratory motion or by interposing a time varying optical beam deflector or by chopping the input radiation. In the case of imaging pyroelectric devices such as TV Cameras with pyroelectric target vidicon tubes, the above techniques have been applied by panning the Camera, by mechanically orbiting an optical element, such as a wedge, a lens, a plate, or by chopping the radiation beam. The final image generated by the camera also varies in time, i.e. it is cyclically displaced in accordance with the panning motion, or with the orbiting motion, or it is time-modulated in accordance with the chopping action. Various techniques have been used in order to produce an image stable in space and time. For example, in panning and in orbiting, cancellation of the image motion is obtained by introducing opposite motion by means of suitable electronic controls; in chopping, cancellation is obtained by introducing complementary image modulation or by recourse to storage techniques, etc. These approaches add complexity and cost to the camera design and reduce its reliability; in the case of chopping, the efficiency is lowered, because only a fraction of the incoming radiation is permitted to reach the detector.
The present invention provides modulation of the target input radiation beam without corresponding displacement or time variation of the camera image. As a result, a stable image is obtained with maximum efficiency and minimum complexity. The invention consists of imparting an axial rotatory motion to the vidicon tube or to the vidicon tube and electronic assembly, exclusive of the yoke assembly, about the rotational symmetry axis of the vidicon tube. An image is generated because of the relative motion between radiation beam and vidicon-tube target; however, since the scanning electronic beam within the vidicon, which depends on the electromagnetic yoke action, does not rotate, the video image remains stable. At the center point of the vidicon target, which does not have, an image is not generated; however, the area involved is very small and the resultant effect on the final image is negligible. The rotatory motion may be continuous or alternating; in the latter case the image is dissolved momentarily at the rotation reversals.